As an effective means for replacing conventional radiography known is a recording and reproducing method of radiation images using stimulable phosphors described in JP-A No. 55-12148 (hereinafter, the term, JP-A refers to an unexamined Japanese Patent Application Publication). In the method, a radiographic image conversion panel (hereinafter, also simply denoted as panel) comprising a stimulable phosphor is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation having passed through an object or having been radiated from an object, sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as “stimulating rays”) to release the radiation energy stored in the phosphor as light emission (stimulated emission), photo-electrically detecting the emitted light to obtain electrical signals, and reproducing the radiation image of the object as a visible image from the electrical signals. The panel, having been read out, is then subjected to image-erasing and prepared for the next photographing cycle. Thus, the radiographic image conversion panel can be used repeatedly.
In the radiation image recording and reproducing methods described above, a radiation image is advantageously obtained with a sufficient amount of information by applying radiation to an object at a considerably smaller dose, as compared to conventional radiography employing a combination of a radiographic film and a radiographic intensifying screen. Further, in conventional radiography, the radiographic film is consumed for every photographing; on the other hand, in this radiation image converting method, in which the radiographic image conversion panel is employed repeatedly, is also advantageous in terms of conservation of resources and overall economic efficiency.
The radiographic image conversion panel employed in the radiation image recording and reproducing method basically comprises a support and provided thereon a phosphor layer (stimulable phosphor layer), provided that, in cases where the phosphor layer is self-supporting, the support is not necessarily required. The stimulable phosphor layer comprises a stimulable phosphor dispersed in a binder. There is also known a stimulable phosphor layer, which is formed by vacuum evaporation or a sintering process, free from a binder, and which comprises an aggregated stimulable phosphor. There is further known a radiographic image conversion panel in which a polymeric material is contained in the openings among the aggregated stimulable phosphor. On the surface of the stimulable phosphor layer (i.e., the surface which is not in contact with the support) is conventionally provided a protective layer comprising a polymeric film or an evaporated inorganic membrane to protect the phosphor layer from chemical deterioration and physical shock.
The stimulable phosphor, after being exposed to radiation, produces stimulated emission upon exposure to the stimulating ray. In practical use, phosphors are employed, which exhibit an emission within a wavelength region of 300 to 500 nm stimulated by stimulating light of wavelengths of 400 to 900 nm. Examples of such stimulable phosphors include rare earth activated alkaline earth metal fluorohalide phosphors described in JP-A Nos. 55-12145, 55-160078, 56-74175, 56-116777, 57-23673, 57-23675, 58-206678, 59-27289, 59-27980, 59-56479 and 59-56480; bivalent europium activated alkaline earth metal fluorohalide phosphors described in JP-A Nos. 59-75200, 6-84381, 60-106752, 60-166379, 60-221483, 60-228592, 60-228593, 61-23679, 61-120882, 61-120883, 61-120885, 61-235486 and 61-235487; rare earth element activated oxyhalide phosphors described in JP-A 59-12144; cerium activated trivalent metal oxyhalide phosphors described in JP-A No. 55-69281; bismuth activated alkaline metal halide phosphors described in JP-A No. 60-70484; bivalent europium activated alkaline earth metal halophosphate phosphors described in JP-A Nos. 60-141783 and 60-157100; bivalent europium activated alkaline earth metal haloborate phosphors described in JP-A No. 60-157099; bivalent europium activated alkaline earth metal hydrogenated halide phosphors described in JP-A 60-217354; cerium activated rare earth complex halide phosphors described in JP-A Nos. 61-21173 and 61-21182; cerium activated rare earth halophosphate phosphors described in JP-A No. 61-40390; bivalent europium activated cesium rubidium halide phosphors described in JP-A No 60-78151; bivalent europium activated cerium halide rubidium phosphors described in JP-A No. 60-78151; bivalent europium activated composite halide phosphors described in JP-A No. 60-78153. Specifically, iodide-containing bivalent europium activated alkaline earth metal fluorohalide phosphors, iodide containing rare earth metal activated oxyhalide phosphors and iodide containing bismuth activated alkaline earth metal halide phosphors exhibited stimulated emission of high luminance.
Along with the spread of radiographic image conversion panels employing stimulable phosphors is further desired an enhancement of radiation image quality, such as enhancement in sharpness and graininess.
The foregoing preparation methods of stimulable phosphors are called a solid phase process or calcination method, in which pulverization after calcination is indispensable, however, there were problems such that it was difficult to control the particle form affecting sensitivity and image performance. Of means for enhancing image quality of radiation images is valid preparation of fine particles of a stimulable phosphor and enhancing particle size uniformity of the fine stimulable phosphor particles, i.e., narrowing the particle size distribution.
Preparation of stimulable phosphors in the liquid phase described in JP-A 7-233369 and 9-291278 is a method of obtaining a stimulable phosphor precursor in the form of fine particles by adjusting the concentration of a phosphor raw material solution, which is valid as a method of preparing stimulable phosphor powder having a narrow particle size distribution. Of rare earth activated alkaline earth metal fluorohalide stimulable phosphors, a phosphor having higher iodide content is preferred in terms of reduction of radiation exposure. This is due to the fact that iodine exhibits a higher X-ray absorption than bromine.
Alkaline earth metal fluoroiodide stimulable phosphors prepared in the liquid phase are advantageous in luminance and graininess but when a precursor thereof is prepared in the liquid phase, the following problems arise. Thus, as described in JP-A 9-291278 and 10-88125, the precursor crystals are prepared in such a manner that: (i) barium iodide is dissolved in water or organic solvents and to the obtained solution, an inorganic iodide solution is added with stirring; or (ii) ammonium fluoride is dissolved in water and to the obtained solution, a barium iodide solution is added with stirring. However, in (i), low barium iodide needs to be present in excess in the solution and the stoichiometric ratio of barium iodide to barium fluoroiodide obtained after solid-liquid separation to added barium iodide often exhibits as small a value as 0.4 or so. Thus, the yield of an alkaline earth metal fluoroiodide stimulable phosphor is often about 40 of the added barium iodide. Even in (ii), excess barium iodide is needed for inorganic fluoride and the yield is also low. Thus, there are problems that the liquid phase synthesis of barium fluoroiodide results in a lower yield, consequently leading to lowered productivity. Reducing the concentration of barium iodide in the mother liquor to enhance the yield results in an increase of particle size, leading to deteriorated image quality.
To enhance the yield of a rare earth activated alkaline earth metal stimulable phosphor, specifically, an alkaline earth metal fluoroiodide stimulable phosphor, JP-A 11-29324 discloses a method for obtaining cubic or rectangular rare earth element-containing barium fluoroiodide crystals having a basic composition of BaFI:xLn (in which Ln: is at least a rare earth element selected from Ce, Pr, Sm, Eu, Gd, Tb, Tm and Yb, 0<x≦0.1) which is obtained by adding a fluorine source to the mother liquor and concentrating the solution.